ream

A quantity of paper, 20 quires. Abbreviation, rm. Well into the 20th
century a ream of writing paper contained 480 sheets and a ream of printing paper
was 500 sheets, while suppliers of printers and publishers used a ream of 516 sheets.
Today the standard ream of all types of paper is 500 sheets.

One sometimes sees it stated that 1 printer’s ream = 21½ quires.1 If so,
these are ordinary 24-sheet quires, not printer’s quires of 25 sheets.